1. Field of the Invention
The present invention relates to a sheet processing apparatus connected downstream of an image forming apparatus that forms an image on a sheet, an image forming system, and an image forming apparatus.
2. Description of the Related Art
Conventionally, there has been known a sheet processing apparatus of a type that punches holes in image-formed sheets, on a sheet-by-sheet basis, while conveying each of them.
In sheet processing apparatuses of the above-mentioned type, a sheet can undergo a shift in position in a direction orthogonal to a sheet conveying direction (the shift will be hereinafter referred to as “lateral registration shift”). To correct such a shift, there has been proposed a sheet processing apparatus that detects a lateral registration shift using a lateral registration-detecting unit, and then moves a punching unit based on a result of the detection, to thereby align a punching position with a target position on the sheet (see e.g. U.S. Pat. No. 5,911,414). Further, there has been proposed another sheet processing apparatus that moves a sheet itself using a sheet shifting unit based on a result of the detection, to thereby align a punching position with a target position on the sheet.
FIGS. 14A and 14B are timing diagrams showing relationship in timing between lateral registration correction and punching. FIG. 14A shows timing in a case where the amount of lateral registration shift is relatively large, and FIG. 14B shows timing in a case where the amount of lateral registration shift is relatively small. It should be noted that each horizontal axis represents time (elapsed time).
In FIGS. 14A and 14B, each of respective operation zones 1201A and 1201B (“A and B” will be hereinafter omitted) represents a time section during which the lateral registration shift of a sheet is detected. An operation zone 1202 represents a time section during which the lateral registration shift detected in the operation zone 1201 is corrected. To correct the lateral registration shift, there have conventionally been proposed a method of moving the punching unit and a method of moving the sheet per se using the sheet shifting unit, as mentioned above, and in the examples illustrated in FIGS. 14A and 14B, the latter method is employed. An operation zone 1203 represents a time section during which the punching unit punches holes in the sheet. Since in the illustrated example, the method of moving the sheet per se using the sheet shifting unit is employed for lateral registration correction, a punching processing execution time period from a time point when the leading end of the shift is conveyed into an area in the sheet processing apparatus for punching processing including lateral registration correction to a time point when the trailing end of the sheet leaves the area after the sheet is subjected to lateral registration correction and punching processing is equal to the sum of a time period required for the sheet to pass through the area and a time period required for performing the punching processing while holding the sheet stationary. Therefore, as shown in FIGS. 14A and 14B, the punching processing execution time period does not change between the case where the amount of lateral registration shift is relatively large and the amount of lateral registration shift is small, but it is constant. An operation zone 1204 represents a time zone during which the sheet shifting unit for lateral registration correction moves to its standby position, and a time period corresponding to the operation zone 1204 is equal to a time period obtained by subtracting the aforementioned punching processing execution time period from a punching processing time period as a total time period concerning a punching process performed by the sheet processing apparatus. A solid line 1205 represents changes in sheet conveying speed, and the vertical axis represents the speed. It should be noted that a horizontal broken line 1206 represents a state where the sheet conveying speed is equal to “0”, i.e. where the sheet is stationary.
When the method of moving the punching unit is employed for lateral registration correction, the punching processing time period becomes different in that at a time point the punching processing executed in the operation zone 1203 is completed, the punching unit is permitted to move to the standby position. However, even when either the method performed by moving the punching unit or the method performed by moving a sheet is employed for lateral registration correction, it takes longer for the punching unit or the sheet shifting unit to return to its standby position as the amount of lateral registration shift is larger, as can be understood from FIGS. 14A and 14B. In other words, as the amount of lateral registration shift is larger, the productivity in sheet processing can become lower.
On the other hand, in recent years, there has appeared on the market an image forming system which is configured such that a plurality of sheet processing apparatuses can be connected downstream of an image forming apparatus so as to perform various kinds of post processing, such as case binding, saddle stitching, folding, and punching. An image forming system of this type can be easily realized in a user's desired one of configurations ranging from a simple one in which only a finisher is connected to an image forming apparatus to a complicated one in which a number of post-processing apparatuses are connected to the image forming apparatus. Further, conventionally, in such an image forming system, a sheet conveying interval is determined such that lateral registration correction for punching can be properly performed even for a maximum lateral registration shift.
By the way, a post-processing apparatus connected upstream of a sheet processing apparatus provided with a punching unit is sometimes equipped with a unit for correcting a lateral registration shift. In this case, the amount of lateral registration shift of a sheet conveyed into the sheet processing apparatus is smaller than in a case where the post-processing apparatus is not equipped with the lateral registration correcting unit. That is, in this case, the distance becomes shorter over which the punching unit or the sheet shifting unit is moved for correction of the lateral registration shift. As a consequence, the punching processing time period (punching processing execution time period including time period required for lateral registration correction+time period required for the sheet shifting unit to return to the standby position) is reduced (see FIG. 14B), which permits reduction of the sheet conveying interval.
However, in the above-described conventional image forming system, the sheet conveying interval is fixed, and it is determined according to a condition that the amount of lateral registration shift is the maximum. In this case, therefore, surplus time unnecessary for operation is produced, which unnecessarily lowers the productivity of sheet processing.
Further, when the number of post-processing apparatuses connected in series upstream of the sheet processing apparatus provided with the punching unit is small, the lateral registration shift of a sheet conveyed into the sheet processing apparatus is smaller than when a larger number of post-processing apparatuses are connected to the sheet processing apparatus. In that case as well, in the conventional image forming system, surplus time unnecessary for operations is produced for the same reason as mentioned above, which unnecessarily lowers the productivity of sheet processing.